Specific features of the transformation of spall cracks to localized shear bands

Investigations of specific features of the microstructure of the region where a spall crack transforms to an adiabatic shear band are based on a spall model of strain localization, which implies that adiabatic shear bands are induced by interference of unloading waves, and the value of the negative stress in the expansion region of these waves does not exceed the dynamic strength of the material. It is shown that the transformation region contains a tremendous number of dislocation ensembles, which is much greater than the number of dislocation ensembles generated by a shock wave. Detection of micrometer-sized fracture sites in the region of interference of unloading waves implies that small fracture sites are formed in a polycrystalline material on dislocations arising in the course of dynamic tension.     

Effect of preloading on the formation of adiabatic localized shear in copper

This paper presents the results of a study of the formation of localized shear in M1 copper of two types: as-received and after preloading by a quasi-entropic compression wave. The experiments were performed with hat-shaped samples using the split Hopkinson bar method. For both types of copper, dynamic compression diagrams were obtained at strain rates of 2100–2500 s^?1. The copper structure was subjected to metallographic analysis, and the effect of preliminary shock deformation on the dynamic mechanical properties of the material was estimated. It is shown that preloaded higher-strength metals with a smaller degree of strain hardening are more prone to the formation of adiabatic shear bands.     

Polymers under mechanical stress: Deformation of the nanostructure of isotactic polypropylene revealed by scanning force microscopy

Scanning force microscopy (SFM) was used to study the lamellar morphology of α and β spherulites in bulk intruded isotactic polypropylene (iPP) samples deformed under shear stress. The iPP specimens were sheared at a constant shear rate at room temperature. Just beyond yielding, at a shear strain level of 0.5, the lamellar structure of the α-spherulites resembles that for unsheared iPP; few cracks are observed along the interfaces between neighboring α-spherulites: this behavior is attributed to a “cross-hatched” lamellar texture of the α-spherulites. In β-spherulites, at the same shear strain, SFM observations reveal the presence of nanocracks perpendicular to the principal tensile strain axis, going through stacks of crystalline lamellae. Profuse local lamellar kinkings occur along the principal compressive strain axis and few nanocracks parallel to the molecular axis are seen in individual lamellae that lie along the shear direction. Neither particular preferred lamellar orientation nor localization of deformation within shear bands has been detected.     

The Mott Fragmentation Model and the Vpl break-up parameter

It is shown that the Mott fragmentation model and the Hirsch formula for the shaped charge jet break-up time complete one another to yield a unified model. This model is consistant with the microscopic picture of internal flow splitting in the metal, into shear bands undergoing a strain larger than the average and the metal between them which undergoes a strain which is smaller than the average. This splitting reduces the deformation energy dissipated by the metal while elongating. It corresponds to the splitting of the stress versus strain characteristics of the metal into the isothermic and adiabatic curves as measured by Johnson and Cook. The parameter V_plwhich describes the break-up process according to the Hirsch model, is identified with the expression (dσ_m/ϱ)^1/2 where dσ_m is the difference in yield strength between the isothermic and adiabatic curves where the adiabatic characteristic becomes a maximum. The comparison with available experimental measurements of this prediction shows a very encouraging agreement. The weak dependence of V_pl on the strain rate on the one hand and the observed increase of the jet break-up time in slowly elongating jets on the other hand are also explained using energy considerations. The recent recovery of jet particles by Zernow yields supportive evidence of the proposed theory which may be extended to all semi steady metal plastic flow phenomena. The final break-up, according to this model is due to the separation between the two sides of shear bands where the slide motion went on during the whole deformation process.     

Damage mechanisms of 2.5D SiO2f/SiO2 woven ceramic matrix composites under compressive impact

Fiber-reinforced SiO_2f/SiO₂ woven ceramic matrix composites (CMCs) are widely applied in the aeronautics and astronautics field due to their many physical and chemical advantages. However, compressive impact loads affect many application scenarios; thus, the damage morphologies and failure modes of these composites under compressive impact should be studied, particularly in the through-thickness direction. In this study, a comparative analysis using the split Hopkinson pressure bar (SHPB) experiment and finite element analysis (FEA) model revealed the damage mechanisms. The compressive impact was simulated in Abaqus/Explicit, and the cohesive element was selected to simulate cohesion of the interface according to the Hashin criterion for multi-mode failure. The results show that 2.5-dimensional SiO_2f/SiO₂ woven CMCs do not have sufficient plasticity to restrain the propagation of micro-cracks under high strain rates after the elastic stage under compressive impact. Many micro-cracks propagated and formed large cracks in the adiabatic shear zone. The adiabatic shear zones were generated when a compressive impact load was applied to the SiO_2f/SiO₂ CMCs at a high strain rate, and these impacts deformed the SiO_2f/SiO₂ CMCs. The adiabatic shear zone occurred at a high strain rate at the weakest point inside the fields of the SiO_2f/SiO₂ CMCs. The micro-cracks propagated and accumulated in this zone. Plastic fracture is the major failure mode for the SiO_2f/SiO₂ CMC specimens; this failure was characterized by fiber yarn fracture and the formation of micro-voids and micro-cracks due to matrix fracture. The large cracks, new voids, and interfacial debonding lead to the failure of the SiO_2f/SiO₂ CMCs. Combined action due to micro-crack formation and its propagation in the adiabatic shear band leads to a softening mechanism of the strain rate.     

Uniaxial compression and combined compression‐and‐shear response of amorphous polycarbonate at high loading rates

We present results of a study conducted to better understand the yield and flow response of amorphous poly(bisphenol A carbonate), PC‐Lexan® (PC), under uniaxial compression and combined compression‐and‐shear impact loading. A split Hopkinson pressure bar (SHPB) is utilized to obtain nearly adiabatic uniaxial compression response of the PC in the strain‐rate range of 1000–2000 s^?1. Since temperature is expected to play an important role in governing the dynamic response of PC, nearly isothermal SHPB tests are also conducted and compared with the adiabatic response. In order to investigate the coupling of shear behavior and dilatation in PC at high loading rates, combined compression‐and‐shear plate impact experiments are conducted at strain‐rates in the range of 10⁵–10⁶ s^?1. In addition, novel plate impact experiments are conducted to better understand the evolution of the shearing resistance of PC in response to sudden alterations (drop) in hydrostatic pressure under extremely high shearing rates. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Effect of the Secondary Transition on the Behaviour of Epoxy Adhesive Joints at High Rates of Loading

The shear yield behaviour of a modified epoxy joint has been investigated over a wide range of strain rates (γ˙ = 10^–2 s^–1 – 10⁴s^–1) and at different temperatures (–30°C, 24°C, 60°C, 80°C).

Assuming that high polymers exhibit pure viscous yield, the sharp increase of the yield stress in the strain rate sensitivity at high strain rates is explained in terms of a difference in relaxation times at low strain rates and high strain rates (α and β). The Bauwens's approach, which is a modification of the Ree-Eyring theory, gives an acceptable fit to the data. The yield behaviour of the modified epoxy joint, above a critical strain rate γ˙_β(T), may be described by the sum of the partial stresses τ_α and τ_β required to free the different kinds of molecular motions implied in the deformation process.

A good correlation between high impact resistance and the presence of the β mechanical loss peak in the range of the explored strain rates is established.

At very low temperature (-30°C), the data do not accurately fit the Ree-Eyring equation, meaning a heterogeneous deformation process characterized by the formation of local adiabatic shear bands and a permanent evolution of the molecular structure.     

Experimental Study on Impact‐induced Initiation Thresholds of Polytetrafluoroethylene/Aluminum Composite

Impact experiments were conducted with a gas gun to investigate the impact‐induced initiation thresholds of a polytetrafluoroethylene/aluminum (73.5 wt % and 26.5 wt %) composite. Targets of steel, aluminum, and low‐density polyethylene materials and sample rods of four different lengths were used to decouple the effects of impact pressure and loading strain rate. By subjecting the samples to different loading conditions, it was shown that the impact‐induced initiation of polytetrafluoroethylene/aluminum is decided by the impact pressure and the loading strain rate simultaneously. The impact pressure and strain‐rate thresholds for initiation were arrested by the experiments. A 30° inclined steel target was used to produce a compression‐shear configuration as a comparison with the normal impact experiments. The initiation was more likely to happen; it demonstrated a shear‐induced initiation mechanism, and a lower initiation strain‐rate threshold was observed under oblique impact. Based on the experimental results, two theoretical curves were proposed to predict the impact‐induced initiation of polytetrafluoroethylene/aluminum under normal and 30° oblique impact.     

B 炸药中绝热剪切带形成机理的细观研究

通过对快速加载后Ｃｏｍｐ．药柱剖面的扫描电镜和光学显微镜观察分析，对Ｃｏｍｐ．Ｂ炸药在快速加载下剪切带形成机理的微观机制有了初步的认识。文中采用形变试样的剖面观察分析方法，针对固体药柱这种比较软的材料，如何进行磨样、抛光、浸蚀，作了一些尝试和探索，总结出一套有效的实施办法。对不同加载速率条件下受压缩的药柱中细观组织进行了观察、分析、比较，首次给出国产Ｃｏｍｐ．Ｂ炸药装药的原始细观组织形貌，以及快速加载后药柱中细观组织形貌。经过分析，得到冲击载荷下炸药装药中绝热剪切带形成、发展的定性认识，为分析Ｃｏｍｐ．Ｂ炸药装药受冲击载荷时热点形成机理提供了一些有益的实验依据。     

Unveiling hermetic failure of ceramic tubes by digital image correlation and acoustic emission

A new, in situ hermeticity testing apparatus has been developed to allow helium leak evaluation of ceramic tubes, including nuclear-grade SiC/SiC fuel cladding ceramic matrix composites (CMC), during four-point bending with simultaneous monitoring of local deformation and damage, using stereoscopic digital image correlation (DIC) and acoustic emissions. The capabilities of the experimental apparatus are demonstrated using alumina, borosilicate glass, and 4130 steel tubes with representative cladding dimensions and then applied to study the deformation-hermeticity relationship of SiC/SiC CMCs. Results of three CMCs appear to indicate that matrix cracking occurs near the deviation from linearity strain at strains ranging from 0.04% to 0.06% and is shortly followed by an initial loss of gas tightness by 0.09% bending strain. Leaking increased in distinct steps over 0.1%-0.2% bending strain, and within this range, results indicate that prior to fiber fracture, it is likely possible to regain gas tightness upon unloading. This technique and uncovered hermetic failure behavior are intended to progress the standardization of a test methodology for nuclear reactor components and to begin to resolve the mechanisms controlling distinct steps of ceramic matrix composite failure.     

The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.     

Kinetics and Mechanisms of High-Temperature Creep in Silicon Carbide: III, Sintered α-Silicon Carbide

The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B₄C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/ T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.     

电机用单组分环氧树脂胶粘剂的研制

通过剪切强度和剥离强度测试研究了不同环氧树脂配比,不同增韧剂及促进剂对单组分电机胶性能的影响。结果表明,CYD-128环氧与双酚F环氧质量比为80∶20,纳米丁腈橡胶粒子VP-501为增韧剂,双氰胺作固化剂,咪唑为促进剂制得的单组分环氧胶25℃和150℃的剪切强度分别达到15.9 MPa,7.3 MPa,可用于电机的低碳钢与磁体的粘接。     

Shear behaviour of structural silicone adhesively bonded steel-glass orthogonal lap joints

This paper outlines an experimental study on the shear behaviour of structural silicone adhesively bonded steel-glass orthogonal lap joints. In the combination of steel plate and glass panel to form a hybrid structural glazing system, bonded joints with structural silicones can provide certain flexibility which relieves stress peaks at critical points of glass panel. The cohesive failure and its related fracture pattern of test joints with varied geometries of adhesives are examined experimentally. It is shown that the presence of two failure modes as discrete voids and macro cracks is closely related to the adhesive thickness. The effects of geometric parameters of adhesives on the joint shear strength are examined. It is demonstrated that the joint shear strengths are increased with increased individual overlap length, reduced adhesive thickness or increased adhesive width while the shear deformation corresponding to maximum shear force is mostly influenced by adhesive thickness. Mechanical contributions for those effects are analyzed accordingly. Finally, an analytical formula allowing for the equilibrium of strain and force on the adhesive and adherend is proposed for the analysis of shear strength. It is demonstrated that calculated normalized shear force ratios predicted by proposed formula agree well with those from experimental results.     

Damage and the nonlinear viscoelastic response of glassy polycarbonate and LaRC-TPI

In the bulk, the nonlinear viscoelastic response of glassy polymers is due to the irreversible work done on the body by the surroundings. The source of the irreversibility is plastic flow of material near distributed shear bands or microcracks in the polymer. Shear bands and microcracks also form new traction free boundaries in the body. The presence of these new boundaries diminish the load bearing capacity of the polymer. These changes in polymer lattice structure are a mechanism that promotes the release of stored strain energy. If the release of stored strain energy is stress controlled, then at sufficiently high levels of stress to cause a permanent structural arrangement of the polymer chains, polycarbonate and LaRC-TPI behave as nonlinear viscoelastic materials. If the current stress is less than the maximum stress the polymer has experienced, then the current energy release rate for the propagation of shear bands, crazes, etc., is less than the critical energy release rate. In this instance, damage production is a constant and the material can be modeled using linear viscoelasticity. It will be shown that the stress-induced nonlinear shift factors are a measure of the rate of damage production in glassy polycarbonate.     

Ni-P化学镀层表面黏液形成菌微生物污垢特性

为了研究化学镀Ni-P换热器上黏液形成菌微生物污垢的特性,利用化学镀Ni-P的方式对低碳钢表面进行改性。采用微生物污垢对比实验,对低碳钢片和具有Ni-P镀层低碳钢片拍摄扫描电镜图,利用称重法记录污垢变化情况和光电比浊法记录黏液形成菌的数量变化情况。结果表明,Ni-P镀层表面形貌明显好于低碳钢;Ni-P镀层相比于碳钢具有很好的耐蚀性和抗微生物污垢特性;黏液形成菌生长繁殖旺盛和细菌代谢产物多时,微生物污垢的形成就快。相反,微生物污垢形成就慢。     

硬质合金覆层-钢基体结合界面层及其生长模型的研究

采用液相烧结技术制备的三元硼化物硬质合金覆层材料具有成本低廉、耐磨抗蚀的优异性能。利用覆层连结试样和抗弯强度法测定的覆层材料的覆层-钢基体界面结合强度为740MPa。覆层与钢基体之间的断裂破坏不是由于覆层与钢基体之间结合界面的剥离,而是发生于界面附近的钢基体和覆层内。利用显微硬度计测定了覆层-钢基体界面结合区的显微硬度变化,并利用SEM-EDS研究了覆层-钢基体界面微观结构和界面区元素分布。在覆层-钢基体结合界面处,存在由高硬度(覆层)到低硬度(钢基体)的狭窄过渡区,合金元素浓度分布没有突变,形成一个具有一定厚度的过渡层。分析了覆层-钢基体界面层形成的机理,并以扩散理论为基础,建立了覆层材料覆层-钢基体界面层生长的数学模型。     

De-painting with high-speed water jets: Paint removal process and substrate surface roughness

Although the use of water jets for paint removal processes is an accepted procedure, there are just a few studies known which discuss parameter optimization and surface topography in some detail. The paper investigates the effects of water jet kinetic energy and stand-off distance on the mass loss of an organic paint system applied to a steel substrate. It was shown that the material removal process was characterized by a combination of loading intensity and loading frequency. Water drops, formed in the water jet at long stand-off distances, played a notable role. For rather high water jet energies, mass loss exhibited high values at high stand-off distances. For lower water jet energies, however, maximum material loss values appeared at a critical stand-off distance. The transition water jet energy was 600 kJ. It could be shown that the steel substrate topography was not compromised due to secondary surface preparation by water jets. Secondary blast cleaning, however, reduced the profile of the substrate.     

De-painting with high-speed water jets: Paint removal process and substrate surface roughness

Although the use of water jets for paint removal processes is an accepted procedure, there are just a few studies known which discuss parameter optimization and surface topography in some detail. The paper investigates the effects of water jet kinetic energy and stand-off distance on the mass loss of an organic paint system applied to a steel substrate. It was shown that the material removal process was characterized by a combination of loading intensity and loading frequency. Water drops, formed in the water jet at long stand-off distances, played a notable role. For rather high water jet energies, mass loss exhibited high values at high stand-off distances. For lower water jet energies, however, maximum material loss values appeared at a critical stand-off distance. The transition water jet energy was 600 kJ. It could be shown that the steel substrate topography was not compromised due to secondary surface preparation by water jets. Secondary blast cleaning, however, reduced the profile of the substrate.     

3,3’-二硝氨基-4,4’-氧化偶氮呋咱羟胺盐的热行为和热安全性研究（英文）

利用C500量热仪研究了3,3′-二硝氨基-4,4′-氧化偶氮呋咱羟胺盐(HNAF)的热分解特性,根据Kissinger和Ozawa方程计算了热分解的动力学参数,同时计算了热分解的热力学参数;采用Micro-DSCⅢ量热仪测定了3,3′-二硝氨基-4,4′-氧化偶氮呋咱羟胺盐的比热容,计算获得了3,3′-二硝氨基-4,4′-氧化偶氮呋咱羟胺盐热安全评价参数。结果表明,HNAF的活化能(E)和指前因子(A)分别为205.26 kJ/mol和1020.32 s-1;活化熵、活化焓和活化吉布斯自由能分别为140.76 J/(mol·K)、201.56 kJ/mol和200.39 kJ/mol。比热容方程与298.15 K时的摩尔比热容分别为C p=-1.560+0.016 T-2.263×10-5 T 2(J/(g·K))和446.028 J/(mol·K)。自加速分解温度、绝热分解温升、热爆炸临界温度分别为444.44 K、2382.89 K、452.86 K,绝热至爆时间为12.46~12.54 s。